Lanthanide Lewis acid catalysts

J. Nishlkido etal, US Patent 6,436,866 (August 20, 2002) Assignee Asahi Kasei Kabushiki Kaisha Utility Lanthanide Lewis Acid Catalysts... 

Kobayashi and coworkers exploited the use of lanthanide Lewis acid catalysts in various achiral reactions as described in the previous section, and they also successfully extended some of them into asymmetric variants. A series of their works commenced with catalytic asymmetric Diels-Alder reactions [50, 51]. The reaction was performed with a chiral ytterbium catalyst prepared from Yb(OTf)3, (R)-l,l -bi-2-naphthol (BINOL), and tertiary amine. The amine significantly influenced enantioselectivity of the reaction, and cis-l,2,6-trimethylpiperidine combined with 4 A molecular sieves (MS 4 A) aflhrded the best results (endo/exo = 89/11, endo = 95% ee, Yb catalyst A) (Scheme 13.20). Later, Nakagawa and coworkers improved reactivity and selectivity of the Yb catalyst by modification of chiral ligand. Use of l,l -(2,2 -bisacylamino)binaphthalene (Yb catalyst B) gave product in >98%ee [52]. 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

To achieve catalytic enantioselective aza Diels-Alder reactions, choice of metal is very important. It has been shown that lanthanide triflates are excellent catalysts for achiral aza Diels-Alder reactions [5]. Although stoichiometric amounts of Lewis acids are often required, a small amount of the triflate effectively catalyzes the reactions. On the basis of these findings chiral lanthanides were used in catalytic asymmetric aza Diels-Alder reactions. The chiral lanthanide Lewis acids were first developed to realize highly enantioselective Diels-Alder reactions of 2-oxazolidin-l-one with dienes [6]. 

Quite a number of asymmetric thiol conjugate addition reactions are known [84], but previous examples of enantioselective thiol conjugate additions were based on the activation of thiol nucleophiles by use of chiral base catalysts such as amino alcohols [85], the lithium thiolate complex of amino bisether [86], and a lanthanide tris(binaphthoxide) [87]. No examples have been reported for the enantioselective thiol conjugate additions through the activation of acceptors by the aid of chiral Lewis acid catalysts. We therefore focussed on the potential of J ,J -DBFOX/ Ph aqua complex catalysts as highly tolerant chiral Lewis acid catalyst in thiol conjugate addition reactions. 

Many Lewis-acid catalysts have been studied and used in the Diels-Alder reactions, ranging from the more commonly used strong Lewis acids such as AICI3, TiCU, SnCU, ZnCli, ZnBri, etc., to the milder lanthanide complexes and to the chiral catalyst. 

As mentioned several times Lewis acids are highly valuable catalysts but the most commonly used ones such as aluminium chloride and boron trifluoride are highly water sensitive and are not usually recovered at the end of a reaction, leading to a significant source of waste. In recent years there has been much research interest in lanthanide triflates (trifluoro-methanesulfonates) as water stable, recyclable Lewis acid catalysts. This unusual water stability opens up the possibility for either carrying out reactions in water or using water to extract and recover the catalyst from the reaction medium. 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

Sc(OTf)3 is stable in water, and effectively activates carbonyl and related compounds as a Lewis acid in water. This is remarkable, because most Lewis acids react immediately with water rather than the substrates, and are decomposed or deactivated. It has already been found that lanthanide trifiates Ln(OTf)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium trifiate Y(OTf)3 are stable in water, and can act as Lewis-acid catalysts in aqueous media.46-48 They are used catalytically in many reactions and can often be recovered and reused, because they are stable under the usual water-quenching conditions. 

S. Kobayashi, H. Ishitani, M Ueno, Facile Synthesis of a-Amino Nitriles Using Lanthanide Triflate as a Lewis Acid Catalyst Synlett. 1997,115-116. 

Lewis acid catalysis. Anhydrous lanthanide(III) chlorides, particularly SmCl3, can function as low-cost but efficient nonhomogeneous Lewis acid catalysts for aldol and other reactions. More rapid reactions are observed when the soluble but expensive Eu(fod)3 is used as a lanthanide catalyst.1... 

Several chiral lanthanide(III) Lewis acid catalysts, derived from chiral binaphthols, have been used in the cycloaddition reactions of cyclopentadiene with substituted iV-acryloyl-1,3-oxazolidin-2-ones. A catalyst derived from ytterbium triflate, (R)-binaphthol... 

The Lewis acidity which is affected by the charge density (Z/r) is less distinct in complexes derived from the large Ln(III) cations. Hence these systems are often reported as mild Lewis acidic catalysts in organic synthesis [41]. However, Sc(III), as the smallest Ln(III) cation by far, is located in a pull position , not only with respect to Lewis acidity, and its aluminum/lanthanide/early... 

Bismuth(m) salts, such as BiCl3, BiBr3, Bi(OCOR)3, Bi(N03)3, Bi(OTf)3, and Bi(NTf2)3, have been used as Lewis acid catalysts to mediate a variety of carbon-carbon bond-forming reactions.85 In some cases, true catalysts differ from the bismuth salts initially added. The most effective and frequently used catalyst is Bi(OTf)3, which is obtained as a hydrated or dehydrated form depending on the preparation methods.86,86a,86b Like lanthanide triflates, Bi(OTf)3 is water stable and reusable. 

Dipolar cycloaddition of alkenes with carbonyl ylides generated in situ is a versatile method for tetrahydrofuran synthesis. The synthetic potential of such transformations has been reviewed <2005JOM(690)5533, 2003BMI6-253>. In addition, the stereoselective [3 + 2] annulation of allyl silanes has become a reliable protocol for the synthesis of tetrahydrofurans as demonstrated in several total syntheses . Such a [3 + 2] annulation, for example, affords the tetrahydrofuran product 11 as a single stereoisomer (Scheme 15) <2002OL2945>. Lanthanide salts serve as efficient Lewis acid catalysts in similar [3 + 2] cycloaddition reactions . 

As mentioned earlier (Section 1.5) another example of novel catalysis in an aqueous medium is the use of lanthanide triflates as water-tolerant Lewis acid catalysts for a variety of organic transformations in water [39]. 

Another approach is to design homogeneous Lewis acids which are water-compatible. For example, triflates of Sc, Y and lanthanides can be prepared in water and are resistant to hydrolysis. Their use as Lewis acid catalysts in aqueous media was pioneered by Kobayashi and coworkers [144-146]. The catalytic activity is dependent on the hydrolysis constant (Kh) and water exchange rate constant (WERC) for substitution of inner sphere water ligands of the metal cation [145]. Active catalysts were found to have pKh values in the range 4-10. Cations having a pKh of less than 4 are easily hydrolyzed while those with a pKh greater than 10 display only weak Lewis acidity. 

Scheme 12) [20a]. Shibasaki et al. [20b] used a chiral in situ generated lanthanide complex (64) as catalyst. The optically active lanthanide complex 66 is postulated as the basic intermediate, activating the nitromethane as shown in 67. However, in the case of the Mukaiyama aldol addition, lanthanide Lewis acids still give moderate ee values. 

Lanthanide salts serve as efficient Lewis acid catalysts in the [3+2] cycloaddition of methylenecyclopropanes with activated aldehydes or ketones (Equation 100) <2003TL3839>. 

Diels-Alder reaction of aldehydes with activated dienes. This lanthanide shift reagent can function as a Lewis acid catalyst in the cyclocondensation of l-methoxy-3-trimethylsilyloxy-1,3-diene (2) with aromatic aldehydes, and permits isolation of the initial... 

It is a commonplace to say that there has been explosive growth in the use of lanthanides in organic chemistry. For many years, the use of cerium(iv) compounds as oxidants was widespread, but more recently a whole range of other compounds have made their appearance. Thus samarium(ii) compounds are now routinely used as one-electron reducing agents and the use of trifluoromethanesulfonate ( triflate ) salts of scandium and the lanthanides as water-soluble Lewis acid catalysts is widespread. Beta-diketonate complexes and alkoxides have also come into use there are even applications of mischmetal in organic synthesis. 

Thorimbert and coworkers have reported the use of complexes (TBA)sH2[al-RE(H20)4 P2W17O61] (RE = La, Sm, Eu, Yb TBA=tetrabutylammonium) as Lewis acid catalysts. These complexes are soluble in organic solvents, and the water molecules on the lanthanide ions are labile, thus providing the metal centers with available coordination sites for organic substrates. These catalysts show high chemoselectivity for the competition reactions between... 

Figure 5.31 Comparison of the chemoselectivity for RE-POM catalysts and RE triflates [90]. (Reproduced with permission from C. Boglio, et al., Lanthanide complexes of the monovacant Dawson polyoxotungstate [al-PiWnOei] as selective and recoverable Lewis acid catalysts, Angewandte Chemie International Edition, 2006, 45, no. 20, 3324-3327. Wiley-VCH Verlag GmbH Co. KGaA.)...

An important discovery in the area of carbonyl cycloadditions is that highly oxygenated 1,3-dienes are excellent 4ir components. These reactions occur at low temperatures with a wide range of aldehydes provided Lewis acid catalysts are used. " " This general type of reaction is shown by the example in equation (81). It was also found by Danishefsky and coworkers that lanthanide shift reagents are elective catalysts for the cycloaddition.An example of one of these cycloadditions is outlined in equation (82). 2 <>... 

How can we generate an even more versatile Lewis-acid catalyst This is probably the ultimate goal of synthetic chemists. The recently developed chemistry of lanthanide catalysts, for example, scandium and ytterbium reagents, might be one recent achievement in this field. The catalyst can even be used in the aqueous phase — obviously not possible with classical Lewis-acid reagents [8]. 

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